The preparation of 6-carboxy-3,4-dihydro-2H-pyran and derivatives of 6-carboxy-3,4-dihydro-2H-pyran has been the subject of a number of investigations.
For example, it is reported by Riobe et al., in C.R. Acad. Sc. Paris Series C, 272, 1045-1048, 1971, that 2,3-dichlorotetrahydropyran can be treated with Cu.sub.2 (CN).sub.2 in the absence of a solvent to give 2-cyano-3-chlorotetrahydropyran which can be converted to 6-cyano-2H-dihydropyran by subsequent treatment with triethylamine. The 6-cyano-2H-dihydropyran product is then converted to 6-carboxy-3,4-dihydro-2H-pyran by hydrolysis with base. Also, Labouc et al., Synthesis, 610-613, 1979, report that 6-lithio-3,4-dihydro-2H-pyrans can be treated with carbon dioxide to form minor amounts of 6-carboxy-3,4-dihydro-2H-pyran.